A team of U.S. anthropologists compared the biomechanics and energy expenditure of both the two-legged and four-legged chimpanzee gaits with a human walk to get a glimpse of what drove the evolution of our bipedal stride.

"The first thing we [saw] off the bat is that humans are really efficient walkers relative to apes," said study co-author Herman Pontzer, a biological anthropologist at Washington University in St. Louis, Missouri.

When it comes to walking, humans use about one-fourth of the energy chimpanzees do, he added.

Chimpanzees, whether they are knuckle walking or upright, have a very crouched gait with bent knees and hips.

Humans, on the other hand, take long steps on straight long legs, requiring less muscle activation—which in turn means less energy spent.

Energy savings could be invested in other activities that ensure survival and improve fitness, such as reproduction and caring for offspring, said William Jungers, an anatomist at New York State's Stony Brook University in Stony Brook who is unaffiliated with the study.

The researchers also found high individual variation in chimpanzee leg length, which impacts the efficiency of the chimp's stride. Such diversity offers a species greater chances of evolving successfully when pressures to adapt—such as walking on two legs—enter the picture.

There could have been that kind of variation among early human ancestors, Pontzer said.

"Evolution needed a foot in the door, and we kind of got a snapshot of that here, which is kind of cool," he said.